Terahertz technologies utilize electromagnetic radiation generally in the frequency range between 100 GHz and 10 THz (i.e, wavelengths of 3 mm to 30 μm, energies of 0.4 to 40 meV, or equivalent blackbody radiation temperatures of 5° K to 500° K). Many non-metallic materials that are visually opaque are partially transparent or exhibit molecular resonances in the terahertz region. In particular, water vapor and other small polar molecules have very strong rotational absorptions at terahertz frequencies. Therefore, terahertz technologies have many potential applications in diverse fields, including molecular spectroscopy, space and atmospheric sciences, plasma physics, biology, medical imaging, remote sensing, and communications.
Historically, there has been much interest in terahertz technologies for high-resolution (i.e., high Q) spectroscopy and remote sensing for space, planetary, and Earth science. For example, much of the interstellar medium radiates in the terahertz region, somewhat above the cosmic microwave background, enabling terahertz measurements to probe star formation and the early universe. Planetary atmospheres have background temperatures of tens to several hundred degrees Kelvin, enabling terahertz observation of extraterrestrial atmospheric conditions. Furthermore, thermal emission lines in the terahertz region from gases in the Earth's stratosphere and upper troposphere provide important indicators of ozone destruction, global warming, and pollution.
However, beyond basic science, applications in the terahertz region are relatively undeveloped. Much progress is still required to provide field-deployable terahertz systems, especially for military, anti-terror, and biomedical imaging applications. Terahertz applications remain relatively undeveloped because the terahertz region lies between the traditional microwave and optical regions of the electromagnetic spectrum, where electronic or photonic technologies have been developed for these purposes. Terahertz applications have been hampered due to the historically poor performance of terahertz components lying in the “technological gap” between these traditional electronic and photonic domains.
Consequently, the generation and detection of electromagnetic fields at terahertz frequencies has been challenging. The weak radiation output from passive (i.e., thermal or backlit by the sun) and active terahertz sources, the low photon energies of terahertz radiation, and high atmospheric attenuation due to molecular absorption (e.g., water vapor) frequently results in a weak received terahertz signal that may be difficult to distinguish from noise. Furthermore, detection of this weak terahertz signal can be difficult.
Current terahertz detectors include both direct and heterodyne detectors. Direct detectors generally directly convert the received power to a voltage or current that is proportional to the incoming power. They are characterized by responsivity (the ratio of the voltage output signal divided by the input signal power, in V/W) and noise-equivalent-power (NEP, the input signal power to the detector required to achieve a signal-to-noise ratio of unity after detection, in W/Hz1/2). A common direct detector uses antenna coupling to a small area Schottky diode that responds directly to the THz electric field. Another direct detector is the conventional bolometer that consists of a radiation absorbing material that is coupled to a sensitive temperature-dependent resistor. For shorter wavelengths (i.e., frequencies above 1 THz), direct detectors generally have good responsivity and are sensitive to a broad band of frequencies. However, they are also sensitive to incoherent background noise and interference. Finally, direct directors are typically very slow, with 1 to 10 ms response times required to obtain an adequate signal-to-noise. Therefore, direct detectors have been used mainly for wideband applications, such as thermal imaging.
There has been an especially strong need for a selective and tunable narrowband detector for spectrum analysis and imaging of the far-infrared (FIR) region of the electromagnetic spectrum (i.e., between 0.1 to 10 THz in frequency). The FIR region contains the spectral signatures of many molecules and materials in vapor and solid phases, ranging from simple diatomic chemicals to complex macromolecules of biological relevance. These signatures arise from molecular resonances that are predominately rotational or hybrid rotational-vibrational modes determined by the molecule's moment-of-inertia. The resulting FIR spectra are far better for molecular discrimination than spectroscopy based simply on mass. In addition, molecular resonance absorption and emission cross-sections in the FIR are generally much larger than at microwave or near-infrared, providing much better sensitivity to relatively small concentrations. Because of this high sensitivity and very high specificity, FIR detectors have enormous potential for chemical and biological sensing. The reliable detection of chemical and biological warfare agents and possible terror agents, such as toxic industrial chemicals, is a current national priority.
Another highly desirable application of terahertz technology is to do imaging of objects at useful standoff distances in real time for object or pattern recognition. Because terahertz irradiation does not involve the health and safety issues of ionizing radiation, such as are a concern with X-ray imaging, applications of terahertz technologies may include noninvasive tomographic imaging or spectroscopic characterization of living materials. Because terahertz radiation is nondestructive and can penetrate non-metallic and non-polarizing external coverings (e.g., clothing, semiconductors, plastics, packaging materials), the technology may be useful in security screening for hidden explosives and concealed weapons. Multispectral, passive FIR imaging is especially desirable for these imaging applications. While imaging can be performed at a single frequency, FIR image contrast between multiple frequencies can provide more information for improved identification.
Existing direct detectors usually respond to total absorbed radiation power, not frequency, and hence require mechanical motion of external optics to generate spectral information. Heterodyne mixers are also used in generating spectral information, but these are tunable only within a narrow range about a specific local oscillator (LO) frequency, and there are very few practical LO sources above about 0.5 THz. The FIR analog of a compact, solid-state microwave or optical spectrum analyzer that continuously covers a broad frequency range with reasonable speed does not yet exist.
Recently, the direct detection of terahertz radiation by two-dimensional (2D) plasma waves has been demonstrated in a double-quantum-well (DQW) field-effect transistor (FET) with a periodic grating gate. See X. G. Peralta et al., Appl. Phys. Lett. 81, 1627 (2002), which is incorporated herein by reference. Coherent charge density oscillations (plasmons) in a high-mobility two-dimensional electron gas (2DEG) can be exploited to circumvent ordinary electronic limits on maximum operating frequency in conventional solid state devices based on electron drift. As a result, the response of the DQW FET can be fast. Also, typical 2DEG densities from 1010 to 1012 cm−2 and device features of 1-10 μm yield plasmon frequencies in the 100 GHz-1 THz range, making plasmon devices attractive for terahertz applications. Furthermore, the ability to electrically tune the 2DEG charge density and hence the plasmon resonance via a gate voltage in the DQW FET enables the detection of specific, user-selected terahertz frequencies.
In FIG. 1 is shown the prior DQW FET 10 of Peralta et al. The DQW FET 10 was fabricated from a modulation doped GaAs/AlGaAs DQW heterostructure grown on a semi-insulating GaAs substrate 11 by molecular beam epitaxy. The two GaAs quantum wells (QWs) 12 and 13 were 20 nm wide and separated by a 7 nm AlGaAs barrier. The nominal electron densities in the QWs 12 and 13 were about 2×1011 cm2. The electron mobility at 4.2° K was about 1.7×106 cm2/Vs. The upper QW 12 was buried 404 nm below the surface of the device. A 2 mm×2 mm mesa was defined by chemical etching and ohmic contacts to both QWs 12 and 13 were formed by evaporating and annealing NiAuGe over the edge and side of the mesa forming the source S and the drain D. A 70 nm thick TiAu grating gate 14 (with no metallization between the grating fingers) was evaporated on the surface of the device with the fingers of the grating parallel to the ohmic contacts, perpendicular to the current flow. The grating period was 4 μm.
As shown in FIG. 2a, the grating spatially modulates the electron density in the QWs 12 and 13 under the metallized part of the gate 14 when a bias voltage Vg is applied to the grating gate, selects wave vectors of the excited plasmon, and produces both normal and transverse THz electric fields. The grating period as well as the channel electron density, under the grating fingers, determine the resonance frequency of the 2D plasma oscillations. THz radiation 15 at frequency f incident on a 2D electron gas of sheet density n will excite plasmon modes when f=fp, according to the dispersion relation
      f    p    =            1              2        ⁢        π              ⁢                  (                                            e              2                        ⁢                          n              j                        ⁢                          k              j                                            2            ⁢                          ɛɛ              0                        ⁢                          m              *                                      )                    1        2            where fp is the resonance frequency, e is the electron charge, kj is the plasmon wavevector, m* is the electron effective mass, and ∈∈0 is the dielectric constant of the semiconductor. In a grating-gated QW FET, n is proportional to (Vg−V0) where V0 is the pinch-off voltage. Therefore, fp is continuously tunable via the grating gate voltage Vg, while k has discrete values set by the gate grating period d: kj=2πj/d, where j=1, 2, 3, . . . is the harmonic mode index.
Prior tuned incoherent direct detectors and related heterodyne mixers based on grating-gated QW FETs have shortcomings for FIR spectroscopy. The DQW FET of Peralta et al. showed low response when Vg was biased on a plasmon resonance, but a steep rise in responsivity, with a concomitant loss of frequency selectivity, was observed for Vg close to V0. Therefore, although the prior DQW FET is fast, it has a low responsivity when operated in the frequency selective mode and loses tunability near pinch-off. Recently, a fast, solid-state heterodyne mixer has been developed based on the grating-gated QW FET. However, as noted previously, heterodyne mixers are tunable only within a narrow range about the LO, and currently there are few practical LO sources that operate above about 0.5 THz. Further, the NEP of the heterodyne mixer may not be adequate for passive, incoherent terahertz imaging. See U.S. application Ser. No. 11/113,635 to Wanke et al.; and M. Lee et al., “Millimeter wave mixing using plasmon and bolometric response in a double-quantum-well field-effect transistor,” Appl. Phys. Lett. 86, 033501 (2005), which are incorporated herein by reference.
Therefore, a need remains for a direct detector that is continuously tunable over a broad terahertz frequency range and has adequate responsivity and sensitivity for spectroscopy and imaging. The detector should preferably be a solid-state device that can be integrated with microelectronic technology to enable a compact spectrometer-on-a-chip, and with terahertz focal plane array technology to enable a passive, multispectral FIR imaging system.